Chromium plating



i 1 means for accomplishing the above purpose.

mium plating various metalobjects. Although the; novel; features which are be- W 1 pendedhereto. the'nature of the invention and its i .jobiects and advantageslmay be better understood i by referring to :the embodiment ithereof which is to Lobtain uniformresults on a. commercial scale 1 pose toovercome these difliculties by utilizing'for whichflthe chromium may bereleasedfiandsxde5 exaans.

n I I have found that electrolytes containin g cer 3 tain soluble higher poiychromates such as tI'I-a c hrornat esv or tetrachroinates'ihavesuch charn acteristicsr llhe presence of a? smallamount oi a suitable catalyzer 'is required for depositing s1 and proportions oi catalyzer may be varied widely n the process. The catalyzerl may; contain various @the deposition of 1 chromium without, combining fere with the plating operation;

g1 the bath to form-compounds'which would interchromate (NagCl's O o) or sodium tetrachromate (NaiCriOn) because these compounds are readily available and may beprepared by welljknown L methods. They may be prepared, forexample, 50 from sodium bichromate and nitricacid, in which the polychromate may be crystallized out present in the solution. The polyehromate may then solved inwater in the proportionsre:

55 lei-plating.

UN T DS SJPATE QI BQ F M n n t v,slohni pltiardick;New Iork, mink].

t No Drawing.-Application March 13, 1935,

h U he Thisilinvention relates togthe electro deposition v qot chromium'fromyanelectrolyte and has iorits object the provision of an improved process and More specifically ther invention 3 relates to an v improved electrolyte or bath for use. inchromium plating and to almethod ofoperation for chro i-lieved to be characteristic of thisinvention will be w ljnore particularly pointed outinthe claims apqmore fully disclosed in the :following description. Successful chromium plating from an electron yte containing chromium is dependent upon van-n ous factors, such as the; nature OfthQQhlOIilillll'l. compound, the nature and the amount oifcatalyaer thevtemperature-ot' the bath andthe current density. The sensitivity of the plating procand ;many timestthe particular factor at fault could not be readily located orcorrected. Iiprothe electrolyte,- 3a chromiumwcompound n-from titles specified have been found to be suitable for Qfchromium from such solutions but thenature i without interfering with the, eflicient operation of i 40 radicals which have the property of promoting 1i withother substanceswhich maybe present in As i a specific example I prefer sodium tri- I have found that solutions contairiingabout 300 "gr; of solublepolychroniate per liter possess the most desirable plating propertiesyunder the conditions to be" specified. meranee'mayf o Outside of about thisranize it isdifiicult to 0btain'abright platei Q ,As specific examples of suitable catalyaers that (maybe used withthe abovefsolutionlthefollow: ing may benot-edz sulphatesy fluoridesand fiuo 1o silicates.

; Anyone of thexabove catalyzersmay used either by itself or in combination with anyother catalyzer."v I'hequantity requireddepends somee what upon the concentration of: the poiychro 15 mate A solution although it may be varied over a comparatively wide range. the case given above, viz: with 300 gr sodiumpolychromate per *literpabout '4 n g; of catalyzer f has been found to pr'oducegood results. wIn: generalthe quantity of catalyzer may varyfrom '5 g. to 15 g. or over per yliter depending uponh the polychromate concentration,wmorecatalyzer being required as the concentration of polychromate is increased.

The following table indicates the range of con- 25 centration of catalyzer that may be used with various quantities of polychromate per liter. It is i H, tozbe understoodWhowever that the upper and posited under aco nparatively wide range of conlower limits may be varied somewhat. The quana bright plate and are given; merely as illustraamperes per square foot. The temperature and current density are not criticalbut the bestresuits have been obtained within about the limits 5 given. i and; separated from other ingredients that may i In the epeiatisnseitmssm s the was to be plated is firsticarefullymbufled, polished and ever beiextend ed from "60 g; to550g. per liter. 5

necessary to use a preliminary coating of a second metal such as copper or nickel. In some cases however it may be more economical to apply a preliminary coating of nickel and/or copper than to clean the suri'ace sufllciently to receive a plate of chromiumdirectly. When properly cleaned base metals such as copp r. brass. n

aluminum and steel may be plated directly from. theabovebath. i t

The article, prepared as above, is placed in the bath as a'cathode'and an insoluble anode, preferably formed 01' a lead and antimony mixture,

is placed in operative relationthereto andp'roperly connected for the passage of electric current through the bath. About 3 or 4 volts are usually required under the conditions above given.

The articles remain in the bath suflicient length of time to receive the proper deposit, usually from .other articles.

2 to 10 minutes according to the desired thickness of the plate and may then be'replaced by Aftercontinueduse the concentration of polychromate will be found ,todecrease due to the chromium removed and deposited at the cathode.

come contaminated by.drag in" oi impurities with the, articles, .to be plated. when the com-- position has been changed in this a manner to suchan extent that a bright platecan no longer be obtainedit is usually desirable to replace the same with a fresh solution. It the quantity of ca alyzer-has been reduced below the bright plate range before the solution is to be. discarded it .may obviously be replaced or strengthened to the required concentration.

Without intending to, limit myself to this explanation, I believe thatthe catalyzer promotes theiormation ottrivalent chromium during the plating process. For example, in the presence oi chromium consisting of a water solution containcatalyzers greenchromium oxysalts, such as chromiumoxysulphatetwhena sulphate catalyzer is used, and some chromium chromate are formed.

' lwould repel'the trivalent chromium compounds from the cathode andinteriere with the deposition of metal. Such excess is avoided however in this process by reason of the low current densities and low temperatures employed.

Some of the trivalent chromium is thus reduced to the metallic form and is deposited at the oathode,and some may bereoxidized at the anode to the hexavalent form. Hence the quantity of these salts remains substantially constant or stabilizes after the bath has been'iri' use for a short time. The required quantity thereof may be added tothe-bath at the beginning of the process if desired, in which-case the catalyzermay be reduced in proportion, as it will not be necessary to form the salts by the-action of the bath. It

is preferred, however, to form the salts in the bath by use of a catalyze! as above explained because a smoother and more easily controlled operation is thereby obtained. In any event a sufficient quantity of catalyzer and/or of the above salts is used to obtain the desired bright plate.

Among the advantages of this process it may be noted that: the plating can be carried on at room temperatures and at low current densities; the polychromate reduces the harmful eilect of impurities such as an excess of sulphates which may accumulate in the bath; the plate is dense and hard; the bath does not require frequent or careful regulation; fuming and spray is reduced to a minimum; and superior throwing power is obtained.

It is obvious that the invention is not to be limited to the exact compositions above mentioned and that various changes and substitutions maybemadebyapersonskilled intheart. A specific embodiment has been given for purposes of illustration only. The invention is accordingly to be limited only inaccordanoe with the following claims when interpreted in view of the prior art.

What is claimed is: Y 1. An electrolyte foruse in chromium plating consisting of a water solution of a soluble polychromate or higher order than a bichromate and a catalyrer. oi the class consisting of sulphates,

fluorides and fluosilicates in proportions suitable i for the electrodeposition of chromium therefrom.

2. An electrolyte for use in chromium plating consisting of a water solution of a solublepolychromate of higher order'than a bichromate and a catalyzer of the class consisting of sulphates. fluorides and fluosilicates in proportions such that a bright plate is obtained at current densities or to 'amperes per square foot and at temperaturesof about:68 degrees F. to 90 de- .grees F;

3. An electrolyte for the electrodeposition of chromium consisting of a water solution containing about 60 g. to 550g. per liter oi sodlun't'trichromate and/or of sodium tetrachromate'and fa catalyzer of the class consisting oi sulphates,

fluorides and fluosilicates.

4. An electrolyte for the electrodeposition of ,-1118 about '60 g. 'to'550 g. per liter of a soluble ;polychromate of higher order than a bichromate and about .5 g. to 15 g. per liter of a catalyzer of the class consisting oi sulphates, fluorides and fluosilicates.

5. An electrolyte for the electrodeposition of I chromium consisting ot-a water solution containing about 60 g. to 550 g. per liter of a soluble polychromate oi higher order than a bichromate and catalyzer of the class consisting of sulphates, fluorides and fluosilicates.

6. The method of chromium plating articles which comprises connecting the articles as a cathode in an electrolyte consisting oi. a water solution containing from about 60 g. to 550 g. per liter of insoluble polychromate of a higher order than a bichromate and a catalyzer or the class consisting of sulphates, fluorides and fluosilicates and passing current therethrough while maintaining the electrolyte at about 68 degrees'l". to 90 degrees F.

7. The methodof chromium plating articles which comprises connecting the articles as a cathode in an electrolyte'consisting of a water solution containing from about 60 g. to 550 g.

per liter of a soluble polychromate of a higher orderthan a bichromate and a catalyzer oi the class consisting of sulphates, fluorides and fluosilicates and passingcuirentthemthroiuh under conditionsof current density and temperature adapted to produce a bright plate.

8."I'he method otchromium plating articles which comprises connecting the articles a: a

cathode in an electrolyte consisting of a water solution containing a soluble polychrcmate of an order hisher than a bichmmate and a catalyzer of the class consisting of sulphates, fluorides andin proportions such that, at the current density and temperature employed. a

throuzh bright plate is produced. passing cun'ent said electrolyte from insoluble anodee and maintaining a current density or less than about 40 amperes per square root 0! cathode surface to be plated anda temperature of about degrees 1".

to9odecreeal'. i JOHNRM IARDICK. 

